A numerical prediction method for internal recirculation and recirculation vortex cavitation in vane pumps

Abstract

The present invention discloses a numerical prediction method for internal reflow and reflow vortex cavitation of a vane pump, which is used to predict the internal reflow and reflow vortex cavitation phenomenon of a vane pump under low flow conditions. The method comprises: step 1) numerically calculating the steady flow inside the pump and the cavitation flow inside the pump under different working conditions in CFD software, extracting flow field information, and obtaining impeller cavitation distribution. Step 2) based on the numerical calculation results of step 1), numerically calculating the unsteady flow inside the pump, and setting relevant pressure pulsation monitoring points. Performing unsteady numerical simulation on typical working points, extracting flow field and pressure pulsation information. Step 3) based on step 2), comparing the main frequency of each monitoring point, and performing phase correlation analysis on the pressure pulsation signal, obtaining the number of unstable flow units and the corresponding axial and radial characteristics. Based on the flow field information of steps 1) and 2), comparing and analyzing the critical working points where reflow and reflow vortex cavitation occur.

Description

Technical Field

[0001] The invention belongs to the technical field related to cavitation of vane-type fluid machinery, and in particular relates to a numerical prediction method for internal reflow and reflow vortex cavitation of a vane pump. Background Art

[0002] Vane pumps are widely used in important national economic fields such as aviation, aerospace, petroleum, chemical industry, water conservancy, etc., with the advantages of simple structure, reliable performance and convenient maintenance. When the pump is operated under conditions that deviate from the design, especially under low flow conditions, the centrifugal pump inlet is prone to backflow, the pressure at the center of the backflow vortex is extremely low, and a low-pressure area is likely to appear in the water suction pipeline, which will cause cavitation.

[0003] Cavitation is common in pump-type rotating machinery and other hydraulic machinery. For pump-type rotating machinery, the impact of cavitation is mostly negative. It is very important to predict cavitation in the early stage of design. The experimental method has problems such as long design cycle and high design cost. Therefore, in order to solve this problem, a numerical prediction method for centrifugal pump backflow and backflow vortex cavitation based on CFD numerical calculation is proposed. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a numerical prediction method for internal recirculation and recirculation vortex cavitation of a vane pump based on computational fluid software, so as to accurately and efficiently predict the recirculation and recirculation vortex cavitation occurring in a vane pump operating under non-design conditions.

[0005] In order to achieve the above objectives, the following calculation scheme is adopted, a numerical prediction method of internal recirculation and recirculation vortex cavitation of a vane pump, comprising the following steps:

[0006] Step 1:

[0007] Step 1) Use the 3D software Creo to perform 3D modeling on the fluid domain of the vane pump and output a model file with the suffix of stp;

[0008] Step 2) Import the three-dimensional model established in step 1) into ICEM, mesh the fluid domain, including: impeller, volute, outlet, inlet and inlet and outlet extensions, use a hexahedral structured grid, add a boundary layer to the structured grid, make the maximum value of y+ value less than 300, check the grid quality, select an appropriate number of grids to ensure calculation accuracy, perform grid independence analysis, and output a grid file with the suffix cfx;

[0009] Step 2:

[0010] Step 3) Import the mesh file from step 2) into ANSYS-CFX, select Steady calculation mode, and set the solver parameters:

[0011] Set the working fluid properties: T = 25℃ pure water, set the density and viscosity coefficient of water at this temperature; set the inlet and outlet boundary conditions: total pressure inlet, mass flow outlet, set the initial inlet pressure boundary condition to 1atm, and change the operating conditions of the pump by setting different outlet mass flow rates; select the isothermal heat transfer model and the RNG k-ε turbulence model; set the speed of the pump impeller; set the wall of the rotating calculation domain to a no-slip wall, set the rotating axis to be consistent with the impeller rotation direction, set the relative velocity of the wall to 0, and use the rotating coordinate system; set the wall of the stationary calculation domain to a no-slip wall, and set it to a global stationary coordinate system; the interface between the dynamic and static calculation domains uses the MRF multiple reference system, the transformation coordinate system is set to Frozen Rotor, and the grid connection method is set to GGI; the static calculation domain interface connection model is set to ordinary connection, and the grid connection method is set to GGI; the convection term, turbulence numerical term, number of solution steps, and calculation convergence condition in the solver parameters are 1e-4, complete the setting of the steady-state numerical calculation of the internal flow of the pump under various conditions, and output it as a def numerical calculation file.

[0012] Step 4) Based on step 3), a steady-state numerical simulation of the internal flow of the pump is performed, and a numerical calculation result file with a suffix of res is output, and at the same time, an external performance curve of the pump is obtained by calculation;

[0013] Step 5) By modifying the turbulence model, pump inlet and outlet boundary conditions, and the length of the pump inlet and outlet pipe extension section, the steady-state numerical calculation method for the internal flow of the vane pump is optimized and verified until the prediction accuracy is achieved, and then proceed to the next step.

[0014] Step 3:

[0015] Step 6) Select the numerical calculation results of the internal reflux flow of the pump at four representative operating points: the design operating point 1.0Q and the small flow operating points 0.85Q, 0.51Q, and 0.34Q.

[0016] Step 7) Based on the steady-state calculation results of the internal recirculation flow of the pump under the working conditions selected in step 6), the distribution information of the pump inlet pipe, the impeller inlet velocity, pressure, vortex, and turbulent kinetic energy is extracted; by setting cross sections at different axial positions in the pump inlet pipe, respectively named as sections 1, 2, 3, 4, 5, and 6, the distances from the impeller inlet surface are 0 mm, 50 mm, 100 mm, 150 mm, 300 mm, and 400 mm, respectively; at the same time, cylindrical surfaces of different radii are set in the pump inlet pipe: d = 1D s d=0.8D s d=0.4D s d=0.2D s .

[0017] Step 8) Compare and analyze the flow field information at different sections under different working conditions, establish the corresponding relationship between each working condition and the degree of backflow development, and obtain the backflow flow field characteristics and backflow occurrence points inside the pump under different working conditions.

[0018] Figure 5 The velocity streamline distribution diagram under different flow rates. Under the working condition of 1.0Q, the streamline of the impeller inlet is relatively smooth, the mainstream inflow fills the entire inlet pipe, and there is no backflow phenomenon. When the flow rate drops to 0.85Q, a small number of irregular streamlines appear on the wall of the impeller inlet pipe, which slightly squeezes the mainstream and causes inlet backflow. When the flow rate drops to 0.51Q, the backflow develops further. When the flow rate continues to drop to 0.34Q, the backflow extends to about 3 times the diameter of the inlet pipe, and the backflow has developed to a relatively intense level.

[0019] Figure 6 The axial velocity and circumferential velocity distribution of section 1 under different flow rates. It can be seen from the figure that at the 1.0Q point where no backflow occurs, the axial velocity at different radii is positive, and the velocity only slightly decreases near r / R1=1.0. At the other flow points where backflow occurs, the velocity value is negative in the range of r / R1 value of 0.8 to 1.0. And as the flow rate continues to decrease, the greater the absolute value of the reverse velocity, the stronger the backflow intensity and the more intense the backflow.

[0020] Step 4:

[0021] Step 9) Add the gaseous medium to the calculation domain, select the homogeneous flow cavitation model; set the mass transfer to Cavitation, and input the saturated vapor pressure of the medium at a certain temperature.

[0022] Step 10) Set the initial volume fraction of the inlet boundary fluid medium, set the volume fraction of the liquid fluid to 1, and the volume fraction of the gaseous fluid to 0.

[0023] Step 11) Setting of inlet and outlet boundary conditions: the inlet total pressure is set to 1 atm and the outlet is set to mass flow rate.

[0024] Step 12) The res file calculated without cavitation inside the pump under the working condition selected in step 6) is used as the initial file, and a cavitation calculation is performed under atmospheric pressure. After the calculation is completed, the inlet pressure of the model pump is gradually reduced to realize the cavitation numerical calculation.

[0025] Step 13) extracting the flow field information of the inlet pipe and impeller inlet under cavitation conditions: such as the distribution of velocity, pressure, vorticity, turbulent kinetic energy, etc.

[0026] Step 14) Obtain the isosurface distribution of cavitation volume in the impeller under different cavitation conditions under different working conditions, that is, under different cavitation coefficients, and obtain the relationship between cavitation development and reflow intensity development. Obtain the isosurface distribution of 10% cavitation in the impeller under different flow rates and the isosurface distribution of 10% cavitation in the impeller under different cavitation coefficients. Obtain the law of cavitation development, and compare and analyze to obtain the relationship between cavitation distribution and reflow intensity development.

[0027] Step 15) For different working conditions, section 1, i.e., the area where backflow occurs, and section 5, i.e., the area where no obvious backflow is observed, are selected to compare and analyze the obtained impeller inlet flow field information and the isosurface distribution of the cavitation volume in the impeller.

[0028] Step 5:

[0029] Step 16) Based on the steady numerical calculation result of the recirculation flow inside the pump under the working condition selected in step 6), as the initial condition, further select a typical working condition point where recirculation occurs but no recirculation vortex cavitation occurs to perform numerical calculation of the unsteady flow inside the pump.

[0030] Step 17) Based on the steady-state numerical calculation result of the pump cavitation flow in step 12) as the initial condition, further select a typical operating point where both backflow and backflow vortex cavitation occur to perform an unsteady numerical simulation of the cavitation flow.

[0031] Step 18) In the unsteady numerical simulation calculation of step 16) and step 17), the turbulence model involved is selected as the RNG k-ε model, the cavitation model is selected as the homogeneous flow model, each rotation of the impeller by 10° is used as the time step of the unsteady calculation, and 30 impeller rotations are used as the total time of the unsteady calculation; the inlet and outlet boundary conditions are selected as the total pressure inlet and the mass flow outlet.

[0032] Step 19) Pressure pulsation monitoring points are respectively set in the inlet pipe, at the impeller inlet, and in the impeller flow channel.

[0033] Step 20) extracting information on the change of the cavitation volume distribution over time of the pump inlet pipe and impeller inlet flow field, and the recirculation vortex cavitation.

[0034] Step 21) Taking one rotation of the impeller as one cycle, select the velocity streamline diagram of the impeller inlet axial section at five times: 0T, 0.25T, 0.5T, 0.75T, and 1T; obtain the dynamic change characteristics of the backflow influence range and the backflow vortex distribution.

[0035] Step 22) Obtain the 10% cavitation isosurface distribution in the impeller and the inlet pipe at different times within one impeller rotation cycle under the selected working condition, and analyze the cavitation distribution at different times to obtain the development process of the recirculation vortex cavitation.

[0036] Step 23) extracts the pressure pulsation signal at the monitoring point set in step 19), and selects the pressure signals of the last 10 cycles in the unsteady calculation results for average value processing.

[0037] Step 24) Perform dimensionless processing on the pressure pulsation signal and define the formula of pressure coefficient as follows:

[0038]

[0039] Where: p is the static pressure at the monitoring point; is the average pressure at the monitoring point during the impeller rotation period; ρ is the density of the liquid; u2 is the circumferential velocity at the impeller outlet.

[0040] Step 25) Perform Fourier transform on the pressure pulsation signal of each monitoring point in MATLAB to obtain the frequency domain diagram of each pressure pulsation monitoring point.

[0041] Step 6:

[0042] Step 26) Based on the calculation results of the pressure pulsation of each monitoring point when the internal recirculation of the pump and the recirculation vortex cavitation occur in step 23), the main frequency of each monitoring point is compared and analyzed.

[0043] Step 27) Perform phase crossover and correlation analysis on the pressure pulsation signals of adjacent monitoring points to obtain the number of unstable flow units in the occurrence and development stages of backflow and backflow vortex cavitation, and obtain their corresponding axial and radial characteristics.

[0044] Step 28) is based on steps 8), 12) and 20) to compare and analyze the flow field information at the inlet pipe when no cavitation occurs and the flow field information at the inlet pipe when cavitation occurs, and determine the critical operating conditions when backflow and backflow vortex cavitation occur; based on the pressure pulsation analysis results of backflow and backflow vortex cavitation in the occurrence and development stages of step 25) and step 27), by comparing the frequency characteristics and radial and axial characteristics of backflow and backflow vortex cavitation; based on the above comparison results, the numerical diagnosis of the inlet backflow and backflow vortex cavitation of the vane pump is realized. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 Schematic diagram of the computational fluid domain of the vane pump in an embodiment of the present invention.

[0046] Figure 2 This is a fluid domain grid diagram in an embodiment of the present invention.

[0047] Figure 3 In the embodiment of the present invention, 1.0Q BEPA、 0.85Q BEPA、 0.34Q BEPA Impeller velocity streamline diagram under flow rate.

[0048] Figure 4 Graphs showing the distribution of axial velocity and circumferential velocity on different cylindrical surfaces in the pump inlet pipe at different flow rates in an embodiment of the present invention.

[0049] Figure 5 It is the distribution diagram of the 10% cavitation isosurface in the impeller at a flow rate of 1.0Q in the example of the present invention.

[0050] Figure 6 The axial and circumferential velocity distributions on section 1 under cavitation conditions.

[0051] Figure 7 1.0Q BEPC Distribution of 10% cavitation isosurface in the impeller under working conditions.

[0052] Figure 8 This is the static pressure distribution with σ=0.81 in section 1 and σ=0.81 in section 5.

[0053] Fig. 9 Distribution of monitoring points at the inlet pipe.

[0054] Fig.10 The impeller inlet velocity streamline diagram at 0.46Q flow rate when σ=0.22, σ=0.15, and σ=0.08 respectively.

[0055] Fig.11 Velocity streamline diagram of the impeller inlet shaft section from 0T to 1T when the operating condition is 0.46Q and the cavitation coefficient σ=0.81.

[0056] Fig.12 0.36Q BEPC , the pressure pulsation frequency domain diagram of the monitoring point on section 1 under the condition of σ=0.82 and the phase cross-analysis diagram of the adjacent monitoring points.

[0057] Fig.13 This is a flow chart of numerically predicting the internal recirculation and recirculation vortex cavitation of a vane pump in an embodiment of the present invention. DETAILED DESCRIPTION

[0058] The present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation modes, but the protection scope of the present invention is not limited thereto.

[0059] This embodiment uses a centrifugal pump as an example, and its main design parameters are: flow rate Q = 94m 3 / h, head H = 30m, speed n = 2950rpmin, conveying medium is clean water, impeller inlet diameter D j=100mm, impeller outlet diameter D2 =1669mm, number of blades Z =7, impeller outlet width b2 =21mm. A closed test bench for the cavitation performance test of a centrifugal pump was built, and the external characteristics test, cavitation performance test and pressure pulsation test of the centrifugal pump were carried out through the closed test bench to obtain the external characteristics, cavitation performance and pressure pulsation test results of the centrifugal pump.

[0060] The numerical diagnosis method for the inlet backflow and backflow vortex cavitation of the vane pump is mainly used to calculate the backflow and backflow cavitation phenomena that occur when the vane pump is running under low flow conditions, and includes the following steps:

[0061] Step (1) 3D model establishment and mesh division:

[0062] Use Creo 3D modeling software to perform 3D modeling on the fluid domain of the centrifugal pump and output the model file with the suffix stp. Figure 1 The three-dimensional modeling of the vane pump is performed; the fluid domain is meshed and the mesh quality is checked. The mesh quality is greater than 0.2 and the minimum mesh angle is greater than 18 degrees. The total number of meshes is 3.02 million. The mesh file with the suffix cfx is output, such as Figure 2 shown.

[0063] Step (2) Import the mesh file of step (1) into ANSYS-CFX and select Steady calculation mode. First, perform the pre-calculation settings:

[0064] Set working fluid properties: T = 25℃ clean water, density 998kg / m 3 ; Select the Isothermal heat transfer model and the k-Epsilon turbulence model; set the inlet and outlet boundary conditions, set the pressure inlet to 1atm, and the flow outlet to 20m 3 / h; set the total pressure inlet (set the initial inlet pressure boundary condition 1atm) and the inlet and outlet boundary conditions of the mass flow rate, and change the operating conditions of the pump by setting different outlet mass flow rates; select the isothermal heat transfer model and the RNG k-ε turbulence model; set the speed of the pump impeller n=2950rpm; set the wall of the rotating calculation domain to a no-slip wall, set the rotating axis to be consistent with the impeller rotation direction, set the relative velocity of the wall to 0, and use the rotating coordinate system; the dynamic and static calculation domain interface adopts the MRF multiple reference system, the transformation coordinate system is set to Frozen Rotor, and the grid connection method is set to GGI; the static calculation domain wall is set to a no-slip wall, the static calculation domain interface connection model is set to ordinary connection, and the grid connection method is set to GGI; the convection term, turbulence numerical term, number of solution steps, and calculation convergence criterion residual in the solver parameters are set to the average RMS value of 10^-4; use the CEL language to write the expression of the head H and efficiency η for monitoring, complete the setting of the steady-state numerical calculation of the internal flow of the pump under various conditions, and finally output it as a def numerical calculation pre-processing file.

[0065] Step (3) performs a steady-state numerical simulation of the internal flow of the pump based on step (2), and outputs a numerical calculation result file with a suffix of res, and at the same time obtains the external characteristic performance curve of the pump through calculation; the steady-state numerical calculation of the internal flow of the vane pump is re-performed by correcting the turbulence model, the pump inlet and outlet boundary conditions, and the length of the pump inlet and outlet pipe extension section, and the calculated values ​​of the external characteristic parameters of the centrifugal pump are compared with their corresponding experimental values, so as to determine whether the selected turbulence model reflects the internal flow state of the pump under this working condition; if the error is greater than 3%, the turbulence model is corrected until the prediction accuracy is achieved.

[0066] Step (4) is based on the steady-state numerical calculation results of the internal flow of the pump in step (3), and the calculation result files of four representative operating points are selected: the design operating point (1.0Q) and the small flow operating points (0.85Q, 0.51Q, 0.34Q); the pump inlet pipe and the impeller inlet flow field information are extracted: such as velocity, pressure, vortex, turbulent kinetic energy distribution, etc.

[0067] Step (5) sets cross sections at different axial positions in the pump inlet pipe, which are named as sections 1, 2, 3, 4, 5, and 6, respectively, and their distances from the impeller inlet surface are 0 mm, 50 mm, 100 mm, 150 mm, 300 mm, and 400 mm, respectively; at the same time, sets cylindrical surfaces of different radii in the pump inlet pipe: d = 1D s d=0.8D s d=0.4D s d=0.2D s; Compare and analyze the flow field information at different cross sections under different working conditions, establish the corresponding relationship between each working condition and the degree of backflow development, and obtain the backflow flow field characteristics and backflow occurrence points inside the pump under different working conditions.

[0068] Step (6) Figure 3 This is the impeller inlet velocity streamline diagram based on the operating point selected in step (4). It can be seen that under the 1.0Q operating condition, the streamlines at the impeller inlet are relatively smooth, the mainstream inflow fills the entire inlet pipe, and no backflow occurs. When the flow rate drops to 0.85Q, a small number of irregular streamlines appear on the impeller inlet pipe wall, which slightly squeezes the mainstream and causes inlet backflow. When the flow rate drops to 0.51Q, the inlet backflow phenomenon is more obvious, and the backflow extends to about twice the pipe diameter upstream of the impeller inlet. The backflow has a significant squeeze on the mainstream. When the flow rate continues to drop to 0.34Q, the backflow extends to about 3 times the inlet pipe, further strengthening the squeeze on the mainstream. In summary, the backflow intensity gradually increases with the decrease in flow rate, and the impact range continues to expand upstream.

[0069] Step (7) Figure 4 The axial velocity and circumferential velocity distribution diagrams of each section under different flow rates, where the positive direction of the axial velocity is defined as the same direction as the mainstream flow direction, and the positive direction of the circumferential velocity is defined as the same direction as the impeller rotation direction. Draw a line graph. According to the velocity streamline diagrams on different cylindrical sections, analyze the flow state on different cylindrical sections when backflow occurs; Figure 6 (a) is the axial velocity distribution diagram on section 1 at different flow rates. It can be seen that at the 1.0Q operating point where no backflow occurs, the axial velocities at different radii are all positive, and the velocity only slightly decreases near r / R1=1.0; while at the other flow points where backflow occurs, the velocity value is negative in the range of r / R1 values ​​of 0.8 to 1.0. And as the flow rate continues to decrease, the absolute value of the reverse velocity increases, which indicates that the backflow intensity increases and the backflow is more intense; by comparing the axial velocity of the axis at different flow rates, it is found that the axial velocity of the axis area does not decrease as the flow rate decreases. This is because the backflow area occupies part of the area of ​​the inlet pipe, which squeezes the mainstream, causing the effective flow area of ​​the mainstream to decrease, so the velocity at the axis position does not decrease significantly; Figure 6(b) is the circumferential velocity distribution diagram on section 1 under different flow rates. It can be seen that under the condition of 1.0Q without backflow, the circumferential velocity is basically 0; but under the condition of backflow, a large circumferential velocity appears in the range of r / R1 value of 0.8 to 1.0, and its value increases with the decrease of flow rate; taking the small flow condition of 0.34Q as an example, the circumferential velocity value at r / R1 = 1.0 is almost 7 times that at the same position under the condition of 1.0Q. In the range of r / R1 value of 0 to 0.3, the circumferential velocity component of the measuring point is basically unchanged, and the circumferential velocity component of the fluid in this area is very small; with the increase of the radius of the measuring point, the circumferential velocity component increases rapidly, and the difference of the circumferential velocity component between different measuring points becomes larger and larger; it can be seen that the flow states near the impeller inlet when backflow occurs are: spiral backflow, spiral inflow, and axial inflow.

[0070] Step (8) adds the gaseous medium vapour to the calculation domain based on the settings of step (2), and selects the homogeneous flow cavitation model; sets the mass transfer to Cavitation, and inputs the saturated vapor pressure of the medium at a certain temperature of 3574Pa; sets the initial volume fraction of the inlet boundary fluid medium, sets the volume fraction of the liquid fluid to 1, and the volume fraction of the gaseous fluid to 0; uses the res file calculated without cavitation as the initial file to perform cavitation calculation under one atmospheric pressure, and gradually reduces the inlet pressure of the model pump after the calculation is completed to realize the cavitation numerical calculation.

[0071] Step (9) is based on the numerical calculation results of step (8), and obtains the 10% cavitation isosurface distribution in the impeller under different flow rates and the 10% cavitation isosurface distribution in the impeller under different cavitation coefficients, so as to obtain the development process of the cavitation inside the impeller. Figure 5 The distribution diagram of 10% cavitation volume value in the impeller at 1.0Q flow rate. When the cavitation coefficient σ = 0.78, cavitation first appears on the back of the blade and the position near the front cover at the inlet edge, and the volume of cavitation on each blade varies greatly; in the process of the cavitation coefficient σ value decreasing from 0.78 to 0.28, the cavitation gradually develops and begins to cover most of the area of ​​the head on the back of the blade. When the cavitation coefficient σ = 0.22, the cavitation has been fully developed in the impeller, not only covering most of the area on the back of the blade, but also blocking all the blade flow passages; when the cavitation coefficient σ = 0.11, the cavitation has been fully developed in the impeller, not only covering most of the area on the back of the blade, but also blocking all the blade flow passages. At this time, the head of the model pump has been seriously reduced; observing the development process of cavitation in the impeller, it can be seen that before cavitation is fully developed in the impeller, the development of cavitation on each blade shows obvious asymmetric and uneven distribution characteristics.

[0072] Step (10) extracts the inlet pipe and impeller inlet flow field information under cavitation conditions based on the numerical calculation results of step (8): such as the distribution of velocity, pressure, vorticity, turbulent kinetic energy, etc. Figure 6 The axial and circumferential velocity distribution on section 1 under cavitation conditions

[0073] Step (11) is based on the numerical calculation results of step (8), obtaining the isosurface distribution of the cavitation volume in the impeller under different cavitation conditions (i.e., different cavitation coefficients) under different working conditions, and obtaining the relationship between the cavitation development and the reflow intensity development. Figure 7 1.0Q BEPB Distribution of 10% cavitation isosurface in the impeller under working conditions.

[0074] Step (12) is based on the numerical calculation results of step (8), and for different working conditions, section 1 (the area where backflow occurs) and section 5 (no obvious backflow) are selected to compare and analyze the obtained impeller inlet flow field information and the isosurface distribution of the cavitation volume in the impeller. Figure 8 Static pressure distribution of different cross sections under different cavitation coefficients.

[0075] Step (13) is based on the steady numerical calculation result of the recirculation flow inside the pump under the working condition selected in step (4) as the initial condition, and further selects a typical working condition point where recirculation occurs but no recirculation vortex cavitation occurs to perform numerical calculation of the unsteady flow inside the pump.

[0076] Step (14) is based on the steady-state numerical calculation result of the pump cavitation flow in step (8) as the initial condition, and further selects a typical operating point where both backflow and backflow vortex cavitation occur to perform an unsteady numerical simulation of the cavitation flow.

[0077] In the unsteady numerical simulation calculation of step (15) in step (4) and step (8), the turbulence model involved is the RNG k-ε model, the cavitation model is the homogeneous flow model, each rotation of the impeller by 10° is used as the time step of the unsteady calculation, and 30 revolutions of the impeller are used as the total time of the unsteady calculation; the inlet and outlet boundary conditions are the total pressure inlet and the mass flow outlet.

[0078] Step (16) Pressure pulsation monitoring points are set in the inlet pipe, at the impeller inlet, and in the impeller flow channel, respectively, such as Fig. 9 As shown. Four points are selected in the inlet pipe, of which P1 and P2 are 350mm away from the impeller inlet, and P2 is located on the axis of the inlet pipe. P3 and P4 are 50mm away from the impeller inlet, and P4 is located on the axis of the inlet pipe; 8 detection points are evenly set on the cross section 1 row, and the angle between adjacent detection points is set to 45°.

[0079] Step (17) extracts information on the change of the pump inlet pipe and impeller inlet flow field, and the cavitation volume distribution of the recirculation vortex cavitation over time, and obtains the development degree of the inlet recirculation under different cavitation coefficients. Fig.10 The figure shows the impeller inlet velocity streamline diagram under different cavitation coefficients at a flow rate of 0.46Q. It can be seen that the inlet return vortex cavitation is generated, developed, and disappeared as the cavitation coefficient σ changes.

[0080] Step (18) takes one rotation of the impeller as one cycle, selects the velocity streamline diagram of the impeller inlet axial section at five moments: 0T, 0.25T, 0.5T, 0.75T, and 1T; and obtains the dynamic change characteristics of the backflow influence range and the backflow vortex distribution.

[0081] Step (19) obtains the 10% cavitation isosurface distribution in the impeller and the inlet pipe at different times within one impeller rotation cycle under the selected working condition, and analyzes the cavitation distribution conditions at different times to obtain the development process of the recirculation vortex cavitation. Fig.11 Velocity streamline diagram of the impeller inlet axial section at different times when the operating condition is 0.46Q and the cavitation coefficient σ=0.81.

[0082] Step (20) extracts the pressure pulsation signal at the monitoring point set in step (19), and selects the pressure signals of the last 10 cycles in the unsteady calculation results for average value processing.

[0083] Step (21) performs dimensionless processing on the pressure pulsation signal and defines the pressure coefficient as follows:

[0084]

[0085] Where: p is the static pressure at the monitoring point; is the average pressure at the monitoring point during the impeller rotation period; ρ is the density of the liquid; u2 is the circumferential velocity at the impeller outlet.

[0086] Step (22) performs Fourier transform on the pressure pulsation signal of each monitoring point in MATLAB to obtain the frequency domain diagram of each pressure pulsation monitoring point.

[0087] Step (23) is based on the pressure pulsation signal processing result of step (22), and performs phase cross-correlation analysis on the main frequencies of adjacent monitoring points, so as to determine the axial or radial characteristics of the unstable flow in the area and obtain whether unstable flow occurs at each frequency. Fig.12The frequency domain diagram and phase analysis diagram of the pressure pulsation monitoring points P5 to P12 on the impeller inlet section 1 under different working conditions and different cavitation coefficients σ. Among them, under the working condition of 0.36Q and the cavitation coefficient of σ=0.82 (no cavitation), the main frequency of each monitoring point is 4 times the shaft frequency; the amplitude of the main frequency of the monitoring points P5 to P12 also shows a trend of first increasing, then decreasing, then increasing and then decreasing, which is related to the unstable flow phenomenon of backflow under this working condition; the secondary main frequency of the monitoring point P10 is 0.1 times the shaft frequency, and the secondary main frequencies of the other monitoring points are all low-frequency frequencies of 0.4 times the shaft frequency; the four main frequencies of 0.11 times the shaft frequency (5.186Hz), 0.12 times the shaft frequency (6.05Hz), 0.39 times the shaft frequency (19.01Hz), and 0.40 times the shaft frequency (19.88Hz) are selected for phase correlation analysis. Fig.12 It can be seen that when the frequencies are 0.11, 0.12, 0.39, and 0.4 times the shaft frequency, the phases are all linearly related, indicating that there is a rotational unstable flow phenomenon corresponding to each frequency at the impeller inlet.

[0088] Step (36) compares and analyzes the flow field information at the inlet pipe when no cavitation occurs and the flow field information at the inlet pipe when cavitation occurs based on steps (9), (12) and (17), and determines the critical operating conditions when backflow and backflow vortex cavitation occur; based on the pressure pulsation analysis results of backflow and backflow vortex cavitation in the occurrence and development stages of steps (19) and (23), the frequency characteristics and radial and axial characteristics of backflow and backflow vortex cavitation are compared; and based on the above comparison results, the numerical diagnosis of the backflow and backflow vortex cavitation at the inlet of the vane pump is realized.

Claims

1. A numerical prediction method for internal recirculation and recirculation vortex cavitation in a vane pump, characterized in that: The following steps are involved: Step 1: Establish the 3D model of the vane pump and divide the mesh: Model the flow field, divide the mesh, check the mesh quality, and perform mesh independence analysis; Step 2: Use CFD software to set the pre-processing file of the steady-state numerical calculation of the internal flow of the vane pump under the design working condition and the small flow condition: import the grid of step 1 into the CFD calculation software, set the pump working fluid properties and the pump inlet and outlet boundary conditions, select the appropriate turbulence model, set the calculation convergence conditions, perform the steady-state numerical calculation of the internal flow of the vane pump, and output the numerical calculation result file; by correcting the turbulence model, the pump inlet and outlet boundary conditions, and the length of the pump inlet and outlet pipe extension section, optimize and verify the steady-state numerical simulation method of the internal flow of the pump until the prediction accuracy is achieved, and then proceed to the next step; Step 3, respectively perform constant numerical calculations of the internal flow of the pump under design conditions and small flow conditions, and obtain the calculation results; Extract the distribution information of pump inlet pipeline, impeller inlet velocity, pressure, vortex, and turbulent kinetic energy under different working conditions; at the same time, obtain the characteristics of the recirculation flow field inside the pump and the recirculation occurrence point; Step 4: Select the homogeneous flow cavitation model in the CFD software, set the inlet total pressure to 1atm, and set the outlet to mass flow rate. Based on the steady numerical calculation results of the non-cavitation flow inside the pump in step 3, gradually reduce the inlet pressure of the vane pump, perform numerical calculations of the cavitation flow inside the pump under the design condition and the small flow condition, obtain the impeller inlet flow field information under different conditions, and the isosurface distribution of the cavitation volume in the impeller under different cavitation conditions, i.e., different cavitation coefficients; Step 5: Based on the steady numerical calculation results of the internal recirculation flow and cavitation flow of the pump in steps 3 and 4, the unsteady flow in the pump is numerically calculated, and relevant pressure pulsation monitoring points are respectively set in the pump inlet pipe, at the impeller inlet and in the impeller flow channel; Select typical operating points in step 3 where recirculation occurs, recirculation vortex cavitation does not occur, and both recirculation and recirculation vortex cavitation occur to perform unsteady numerical simulation; Extract the information about the change of cavitation volume distribution over time in the pump inlet pipe, impeller inlet flow field, and recirculation vortex cavitation; extract the pressure pulsation results at the above monitoring points, and perform frequency domain analysis of pressure pulsation at each monitoring point by programming in MATLAB; Step 6: Based on the pressure pulsation calculation results of each monitoring point when the internal recirculation and recirculation vortex cavitation of the pump occur in step 5, by comparing and analyzing the main frequency of each monitoring point, and performing phase correlation analysis on the pressure pulsation signals of monitoring points at different positions, the number of unstable flow units in the occurrence and development stage of recirculation and recirculation vortex cavitation is obtained, and the corresponding axial and radial characteristics are obtained; Based on the flow field information when the internal recirculation and flow vortex cavitation of the pump occur in steps three, four, and five, the critical operating points of the recirculation and recirculation vortex cavitation are compared and analyzed.

2. The numerical prediction method for internal recirculation and recirculation vortex cavitation of a vane pump according to claim 1, characterized in that: The specific steps of step one are: Step 1) Use the 3D software Creo to perform 3D modeling on the computational fluid domain of the vane pump and output a model file with the suffix of stp; Step 2) Based on ICEM CFD software, use hexahedral structured grids to mesh the computational fluid domains of the pump. At the same time, add boundary layers to the generated structured grids so that their maximum y+ value is less than 200. Check the grid quality, select the appropriate number of grids to ensure the calculation accuracy, perform grid independence analysis, and output the grid file with the suffix cfx.

3. The numerical prediction method for internal recirculation and recirculation vortex cavitation of a vane pump according to claim 2, characterized in that: The step 2 specifically includes the following steps: Step 3) Import the mesh file of step 2) into ANSYS-CFX, select Steady calculation mode, and set the solver parameters: set the working fluid properties: T = 25℃ pure water, set the density and viscosity coefficient of water at this temperature; set the inlet and outlet boundary conditions: total pressure inlet, mass flow outlet, set the initial inlet pressure boundary condition to 1atm, and change the operating conditions of the pump by setting different outlet mass flow rates; select the isothermal heat transfer model and the RNG k-ε turbulence model; set the speed of the pump impeller; set the wall of the rotating calculation domain to a no-slip wall, set the rotation axis to be consistent with the impeller rotation direction, set the relative velocity of the wall to 0, and use the rotating coordinate system; set the wall of the stationary calculation domain to a no-slip wall, and set it to a global stationary coordinate system; the interface between the dynamic and static calculation domains uses the MRF multiple reference system, and the transformation coordinate system is set to Frozen Rotor, the grid connection mode is set to GGI; the static calculation domain interface connection model is set to ordinary connection, and the grid connection mode is set to GGI; the convection term, turbulence numerical term, solution step number, and calculation convergence condition in the solver parameters are set to 1e-4, and the setting of the steady-state numerical calculation of the internal flow of the pump under various working conditions is completed, and the numerical calculation file is output as def; Step 4) Based on step 3), a steady-state numerical simulation of the internal flow of the pump is performed, and a numerical calculation result file with a suffix of res is output, and at the same time, an external performance curve of the pump is obtained by calculation; Step 5) Based on step 3), the turbulence model, the pump inlet and outlet boundary conditions, and the length of the pump inlet and outlet pipe extension section are corrected to recalculate the constant values ​​of the internal flow of the vane pump, and the calculated values ​​of the external characteristic parameters of the centrifugal pump are compared with their corresponding experimental values ​​to determine whether the selected turbulence model is consistent with the internal flow state of the pump under the working condition; if the error is greater than 3%, the turbulence model is corrected until the prediction accuracy is achieved.

4. The method for numerical prediction of internal recirculation and recirculation vortex cavitation in a vane pump according to claim 3, characterized in that: The step three specifically includes the following steps: Step 6) Select the numerical calculation results of the internal reflux flow of the pump at four representative operating points: the design operating point 1.0Q and the small flow operating points 0.85Q, 0.51Q, and 0.34Q; Step 7) Based on the numerical calculation results of the internal reflux flow of the pump under the working condition selected in step 6), the distribution information of the pump inlet pipe, the impeller inlet velocity, pressure, vortex, and turbulent kinetic energy is extracted; by setting cross sections at different axial positions in the pump inlet pipe, respectively named as sections 1, 2, 3, 4, 5, and 6, the distances from the impeller inlet surface are 0 mm, 50 mm, 100 mm, 150 mm, 300 mm, and 400 mm, respectively; at the same time, cylindrical surfaces of different radii are set in the pump inlet pipe: d = 1D s d=0.8D s d=0.4D s d=0.2D s ; Step 8) Based on step 7), comparative analysis is performed on the flow field information at different working conditions and different cross sections to establish a corresponding relationship between each working condition and the degree of reflux development, and at the same time, the reflux flow field characteristics and reflux occurrence points inside the pump under different working conditions are obtained.

5. The numerical prediction method for internal recirculation and recirculation vortex cavitation of a vane pump according to claim 4, characterized in that: The step 4 specifically includes the following steps: Step 9) Based on the settings of step 3), the gaseous medium is added to the calculation domain, and the homogeneous flow cavitation model is selected; the mass transfer is set to Cavitation, and the saturated vapor pressure of the medium at a certain temperature is input; Step 10) setting the initial volume fraction of the inlet boundary fluid medium, setting the volume fraction of the liquid fluid to 1 and the volume fraction of the gaseous fluid to 0; Step 11) Setting the inlet and outlet boundary conditions: the inlet total pressure is set to 1 atm, and the outlet is set to mass flow; Step 12) using the res file of the calculation result of the steady flow without cavitation inside the pump under the working condition selected in step 6) as the initial file of the steady numerical calculation of the cavitation flow of the pump, firstly performing the cavitation calculation under the condition that the pump inlet is at one atmospheric pressure, and gradually reducing the pump inlet pressure in sequence after the calculation is completed to realize the cavitation numerical calculation; Step 13) obtaining flow field information of the inlet pipe and the impeller inlet under different working conditions and different cavitation conditions, wherein the flow field information includes velocity, pressure, vorticity, and turbulent kinetic energy distribution; Step 14) obtaining the isosurface distribution of the cavitation volume in the impeller under different working conditions and different cavitation conditions, i.e., different cavitation coefficients, and obtaining the relationship between the cavitation development and the reflow intensity development; Step 15) For different working conditions, select section 1, i.e., the area where backflow occurs, and section 5, i.e., the area where no obvious backflow is observed, and compare and analyze the obtained impeller inlet flow field information and the isosurface distribution of the cavitation volume in the impeller.

6. The method for numerical prediction of internal recirculation and recirculation vortex cavitation in a vane pump according to claim 5, characterized in that: The step five specifically includes the following steps: Step 16) based on the steady numerical calculation result of the pump internal recirculation flow under the working condition selected in step 6), as the initial condition, further selecting a typical working condition point where recirculation occurs but recirculation vortex cavitation does not occur, and performing numerical calculation of the unsteady flow inside the pump; Step 17) based on the steady numerical calculation result of the pump cavitation flow in step 12) as the initial condition, further selecting a typical operating point where both backflow and backflow vortex cavitation occur, and performing an unsteady numerical simulation of the cavitation flow; Step 18) In the unsteady numerical simulation calculation of step 16) and step 17), the turbulence model involved is selected as the RNGk-ε model, the cavitation model is selected as the homogeneous flow model, each rotation of the impeller by 10° is used as the time step of the unsteady calculation, and 30 revolutions of the impeller are used as the total time of the unsteady calculation; the inlet and outlet boundary conditions are selected as the total pressure inlet and the mass flow outlet; Step 19) setting pressure pulsation monitoring points in the inlet pipe, at the impeller inlet, and in the impeller flow channel; Step 20) extracting information on the change of the cavitation volume distribution of the pump inlet pipe and the impeller inlet flow field and the backflow vortex cavitation over time; Step 21) Taking one rotation of the impeller as one cycle, select velocity streamline diagrams of the impeller inlet axial section at five times: 0T, 0.25T, 0.5T, 0.75T, and 1T; obtain the dynamic change characteristics of the backflow influence range and the backflow vortex distribution; Step 22) obtaining the 10% cavitation isosurface distribution in the impeller and the inlet pipe at different times within one impeller rotation cycle under the selected working condition, and analyzing the cavitation distribution at different times to obtain the development process of the backflow vortex cavitation; Step 23) extracting the pressure pulsation signal at the monitoring point set in step 19), selecting the pressure signals of the last 10 cycles in the unsteady calculation results for average value processing; Step 24) Perform dimensionless processing on the pressure pulsation signal and define the formula of pressure coefficient as follows: Where: p is the static pressure at the monitoring point; is the average pressure at the monitoring point during the impeller rotation period; ρ is the density of the liquid; u2 is the circumferential speed at the impeller outlet; Step 25) Perform Fourier transform on the pressure pulsation signal of each monitoring point in MATLAB to obtain the frequency domain diagram of each pressure pulsation monitoring point.

7. The numerical prediction method for internal recirculation and recirculation vortex cavitation of a vane pump according to claim 6, characterized in that: The step six specifically includes the following steps: Step 26) based on the calculation results of the pressure pulsation of each monitoring point when the internal recirculation of the pump and the recirculation vortex cavitation occur in step 23), the main frequency of each monitoring point is compared and analyzed; Step 27) performing phase crossover and correlation analysis on the pressure pulsation signals of adjacent monitoring points, obtaining the number of unstable flow units in the occurrence and development stages of backflow and backflow vortex cavitation, and obtaining the corresponding axial and radial characteristics; Step 28) is based on steps 8), 12) and 20) to compare and analyze the flow field information at the inlet pipe when no cavitation occurs and the flow field information at the inlet pipe when cavitation occurs, and determine the critical operating conditions when backflow and backflow vortex cavitation occur; based on the pressure pulsation analysis results of backflow and backflow vortex cavitation in the occurrence and development stages of step 25) and step 27), by comparing the frequency characteristics and radial and axial characteristics of backflow and backflow vortex cavitation; based on the above comparison results, the numerical diagnosis of the inlet backflow and backflow vortex cavitation of the vane pump is realized.

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